Transformation of β to α phase of isotactic polypropylene nucleated with nano styrene butadiene rubber-based β-nucleating agent under microwave irradiation

Abstract

This paper investigates the effect of microwave irradiation on the β to α phase transformation of the β-nucleated isotactic polypropylene (iPP). Ten microwave irradiation cycles was applied to the iPP and iPP modified with 0.3 wt% and 0.5 wt% β-NA, and the data at 2nd, 4th, 6th, 8th and 10th irradiation were reported. As expected, the sample temperature was found to increase with the irradiation time, by more than 130 °C, due to high frequency of microwave processing. This was the major factor that induced the β-phase transformation and structural change. Both the differential scanning calorimetry (DSC) and X-ray diffraction (XRD) results indicated that β-phase was mainly transformed to α-phase and partially converted to the amorphous section. It was reflected as 1) the reduction of the enthalpy of β-crystal melting (ΔHmβ), 2) the increased enthalpy of α-crystal melting (ΔHmα), 3) the decreased β-crystalline phase fraction (Kβ) and 4) the decrease of the overall degree of crystallinity (Xall). Under impact force, neat iPP showed a slight increase in the impact strength with the irradiation time, due to the increase of amorphous region. For the β-iPP, it decreased due to the reduction of the β-phase content.